My energy level can fluctuate significantly during the day and from day to day.  I like to try different exercises to keep my energy level as balanced as possible.. Here's a pleasant exercise you can do to re-balance your energy centers (called chakras) using your voice to tone a sound that resonates with the seven major energy centers in the body.  Bija means jewel or seed.  This exercise balances your chakras using Bija mantras.
Base Chakra: Sound of LAM (sounding like "larm"), note of deep C
Sacral Chakra: Sound of VAM (sounding like "varm"), note of D
Solar Plexus Chakra: Sound of RAM (sounding like "rarm"), note of E
Heart Chakra: Sound of YAM (sounding like "yarm"), note of F
Throat Chakra: Sound of HAM (sounding like "harm"), note of G
Brow Chakra: Sound of OM (sounding like "A" "U" "M"), note of A
Crown Chakra: Sound of OM (sounding like "A" "U" "M"), note of B
If you are unsure about the musical notes, just sound the mantras intuitively in an ascending scale with the intent of having a healing, balancing sound.  Source:  The Chakra Bible by Patricia Mercier.